One of Juliet's primary traits is her ever-present impatience. In part, perhaps, because of her age, Juliet seems incapable of allowing events to develop at their own pace and, instead, consistently tries to force various issues throughout the play.

For example, in Act IV, scene i, when Juliet discusses Romeo's banishment with Friar Lawrence, she states:

Give me some present counsel, or, behold,
'Twixt my extremes and me this bloody knife
Shall play the umpire, arbitrating that
Which the commission of thy years and art
Could to no issue of true honour bring.
Be not so long to speak; I long to die,

We see here Juliet's impatience and her eagerness to take matters not only into her own hands but also to extremes.

In fact, Juliet comments on her own impatience during her soliloquy in Act III, scene ii:

O, I have bought the mansion of a love,
But not possess'd it, and, though I am sold,
Not yet enjoy'd: so tedious is this day
As is the night before some festival
To an impatient child that hath new robes
And may not wear them.

A dominant character trait of Juliet, which is visible in Acts 1 through 3, is that she is quick to recognize and adapt to new circumstances and situations. At the best of times, this is resourcefulness. At the worst of times, this is impetuousness.

The first time this is seen is at the party following her exchange with her mother. She tells her mother that she will "do her will" and agrees to meet Paris and consider his proposal of marriage, even promising to govern her acquaintance of him by her mother's wishes. At the party, things change.

Juliet's age is identified as being just under fourteen and she deports herself with decorum and dignity, though we also know that her meeting with Paris will be her first association with courtship. Then she encounters Romeo.

ROMEO: [To JULIET] If I profane with my unworthiest hand
This holy shrine, the gentle fine is this:
My lips, two blushing pilgrims, ready stand
To smooth that rough touch with a tender kiss.

She quickly recognizes a new situation and readily adapts to it with resourcefulness by offering a little encouraging flirtation of her own, while keeping pace with Romeo's allusions to Christian pilgrims:

JULIET: Good pilgrim, you do wrong your hand too much,
Which mannerly devotion shows in this;
For saints have hands that pilgrims' hands do touch,
And palm to palm is holy palmers' kiss.

Later, when Romeo breaches the orchard wall and they romance each other under the light of the changeful orb, the moon, "the inconstant moon, / That monthly changes in her circled orb," Juliet recognizes another new situation and quickly, though impetuously, adapts to it by saying that if Romeo has an honorable purpose to his love and intends marriage between them, she will meet him on the morrow to perform the rite of marriage:

JULIET: If that thy bent of love be honourable,
Thy purpose marriage, send me word to-morrow,
By one that I'll procure to come to thee,
Where and what time thou wilt perform the rite;

These are but two of the instances in which Juliet meets and recognizes a new situation to which she quickly adapts. With these two examples in hand, you'll be able to identify others and determine which are resourceful and which are impetuous.

To me, Juliet's major character trait is that she's fourteen.  I know, that's not a character trait, but it leads to one.  I think her major character trait is that she is overdramatic and gets too caught up in her new love.  I think that if either she or Romeo had been a bit more mature, things could have turned out a lot better.  Here are a couple of quotes that could support this:

The first is from Act I, Scene 5.  Romeo has just left and Juliet is head over heels.  She says:

Go ask his name: if he be married.
My grave is like to be my wedding bed.

Sounds dramatic, huh?  If he's married, I'm totally going to die, OMG!!

Another one could be from Act II, Scene 5.  She's sent the nurse off to talk to Romeo.  She's back and she's all complaining that she's tired and sore.  But Juliet can't wait.  She says, for example

I' faith, I am sorry that thou art not well.
Sweet, sweet, sweet nurse, tell me, what says my love?

Yeah, yeah, I know you're tired.  So what did Romeo say??